--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -75,17 +75,17 @@ def idlTypeNeedsCycleCollection(type):
     type = type.unroll()  # Takes care of sequences and nullables
     if ((type.isPrimitive() and type.tag() in builtinNames) or
         type.isEnum() or
         type.isString() or
         type.isAny() or
         type.isObject() or
         type.isSpiderMonkeyInterface()):
         return False
-    elif type.isCallback() or type.isGeckoInterface():
+    elif type.isCallback() or type.isPromise() or type.isGeckoInterface():
         return True
     elif type.isUnion():
         return any(idlTypeNeedsCycleCollection(t) for t in type.flatMemberTypes)
     elif type.isRecord():
         if idlTypeNeedsCycleCollection(type.inner):
             raise TypeError("Cycle collection for type %s is not supported" % type)
         return False
     elif type.isDictionary():